Extreme ultraviolet (EUV) radiation with wavelengths of λ<50 nm which is needed, particularly at wavelengths λ≦13.5 nm, for photolithography fabrication of next-generation integrated circuits can be generated by plasma-based radiation sources. The EUV radiation can be generated through excitation of a suitable source material whose plasma has emission lines in the EUV spectrum. For plasma generation, the source material must be excited inside a radiation source, the generated EUV radiation then being coupled out of the latter. The two primarily accepted methods for plasma generation in the wavelength range around 13.5 nm are excitation by means of high-energy radiation, e.g., by means of a laser beam (Laser-Produced Plasma—LPP) or by means of a gas discharge (Discharge-Produced Plasma—DPP) and hybrid forms thereof, among which Laser-assisted Discharge Plasma (LDA) has become most prevalent. Plasma generation is an energy-intensive process in which an efficiency of about 0.1% is achieved. The majority of the energy that is used is lost in the form of waste heat. In order to dissipate the very large amounts of waste heat, a radiation source of this kind requires an efficient cooling system. High cooling efficiency can be achieved through the use of a metal coolant. This metal coolant has a high heat capacity coupled with high heat conductivity so that large amounts of heat can be dissipated quickly.
In radiation sources in which plasma formation takes place by means of gas discharge, two electrodes are located opposite one another and a strong electric field is produced locally therebetween. The source material is introduced into the electric field in a form which leads to the gas discharge.
A radiation source of this type is disclosed in EP 1 804 556 A2, wherein the two electrodes are arranged as circular disk electrodes in a plane so as to be rotatable around their orthogonal axes of rotation such that they have at a point on their outer circumference an electrode gap with minimal distance relative to one another. The gas discharge takes place in this electrode gap.
The source material takes the form—separately for each disk electrode—of a melt bath with suitable metal into which the disk electrode is dipped by a portion of its circumference. In order to keep the metal in liquid state, it is heated in the radiation source to at least above its melting point. As the disk electrode rotates through the melt bath, a thin layer of metal forms at the circumference and is conveyed into the electrode gap by the rotating disk electrode.
In the electrode gap, a laser beam by which the source material is initially evaporated is directed to one of the disk electrodes. Accordingly, evaporated, partially ionized source material is present in the electrode gap and is converted into completely ionized, hot plasma by a subsequent gas discharge (LDP). The plasma developing in this way emits the desired EUV radiation.
During continuous operation of the radiation source, the disk electrodes are highly heated. For cooling the disk electrodes, EP 1 804 556 A2 discloses also using the source material as metal coolant. In so doing, the waste heat is passed to the melt bath coming in contact with the rotating disk electrode, and the metal coolant is additionally set in motion through the rotation of the disk electrode so that a continual exchange of the liquid metal coming in direct contact with the disk electrodes takes place in the entire volume of the melt bath. Steps are described for enhancing the circulation of the liquid metal by arranging radial ribs or holes at the disk electrodes. However, no details are given as to how to correct the temperature of the liquid metal bath in a suitable manner to ensure trouble-free functioning during continuous operation as well as during process-related interruptions in radiation.
A possibility for cooling the liquid metal that is used as source material and as coolant simultaneously is disclosed in the not-prior-published DE 10 2013 103 668 which, in addition to the melt bath for the disk electrode, describes a handling device for the liquid metal for producing a cooling circuit. A compact source module with disk electrode and melt bath is connected in a circuit with the handling device via feed/return conduits and provides for a pump-assisted circulation. Owing to the compact construction of the source module, the handling device has a reservoir in which the greatest proportion of the liquid metal present in the circuit is located. The temperature of the liquid metal can be corrected to an optimal temperature just above its melting point in the handling device. To this end, an additional cooling device is also connected to the handling device in order to keep the liquid metal that is pumped from the reservoir back to the source module in the circuit at a suitable temperature through active cooling of a return pipe. Spray cooling is suggested by way of example.
Many scientific articles have already been published on the subject of applying spray cooling. An overview is given by Jungho Kim in the article: “Spray cooling heat transfer: The state of the art” (International Journal of Heat and Fluid Flow 28/2007). Using applications in high-power electronics as an example, the article describes mechanisms and influencing factors of spray cooling which were studied for laminar cooling of small surfaces in the range of a few square centimeters. Further, another survey article is known in which, inter alia, the cooling performance of nozzle arrays was investigated (Yan, Z. B. et al., “Large area spray cooling by inclined nozzles for electronic board”—12th Electronics Packaging Technology Conference 2010).
Common to all of the publications is that, apart from maximizing cooling performance, no measures are described for a load-dependent operation of cooling nozzles of a spray cooling arrangement under sharply fluctuating heat input.